That's the point. If your opponent DIDN'T play anything on his turn and you have counter mana still available, you still have the option to use that mana for something before starting your turn. I use them in my current deck and sometimes will go over 7 cards in my hand at the end of my opponent's turn, and then cast down below the 7 card limit on my turn so I don't have to discard. But if you want to use Dark Tutor to pull up specific cards as opposed to gaining general card advantage, that's entirely your prerogative. :) I just don't like having to tap out to use it. Nice deck for sure, though.
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Jace's Ingenuity isn't bad in this type of deck either, lets you refill your hand at the end of their turn if you still have counter mana up that didn't get used.